Plasma spraying devices for spraying heat fusible materials have proven effective for surface treatment and coating applications. Generally, plasma spraying devices operate by first generating a plasma discharge and then introducing a heat-fusible material into the plasma. A resultant spray of plasma and material is discharged through a nozzle in the form of a plasma jet.
Plasma discharges can be generated in various ways. Conventional plasma spraying devices utilize direct current (hereinafter "DC") plasma discharges. To create a DC plasma discharge, a potential is applied between two electrodes, a cathode and an anode, in a gas. A resulting current passing through the gas excites the gas molecules, thereby creating a plasma discharge. Once a discharge is formed, most of the space between the cathode and anode is filled by a plasma discharge glow. A comparatively dark region forms adjacent to the cathode corresponding to the cathode plasma sheath. A similar dark region forms adjacent the anode, but it is very thin compared to the cathode dark region.
The interaction between the plasma and the electrodes eventually results in erosion of the electrodes. In addition, the interaction between the plasma and the electrodes results in the deposition of some heat-fusible material on the electrodes.
DC plasma discharges can result in unstable operation which may make it difficult to strike and maintain the plasma. Also, the unstable operation may result in nonuniform plasma spraying.
Radio frequency (RF)-driven plasma sprayers have been developed to overcome problems inherent to DC plasma discharge sprayers. Prior art microwave-driven plasma sprayers utilize plasma discharge tubes formed of dielectric material to confine the plasma. Some RF-driven plasma sprayers utilize small diameter discharge tubes to encourage gas circulation at a low flow rate.
Discharge tubes formed of dielectric material are limited in the microwave powers they can withstand. In addition, because of the interaction between the plasma and the dielectric tube, some heat-fusible material deposits on the tube. Deposits of heat-fusible material on the dielectric tube contaminate the sprayer and cause unstable operation which may result in nonuniform plasma spraying.
It is therefore a principal object of this invention to provide a microwave-driven plasma sprayer without a discharge tube which can be utilized for uniform high-powered plasma spraying. It is another object of this invention to provide a plasma sprayer relatively free of contamination caused by deposits of heat-fusible material. It is another object of this invention to provide a plasma sprayer which generates a uniform plasma spray.